Electric motors are typically used to affirmatively drive structures from one position to another position. However, in some applications, electric motors are also used as brake mechanisms to hold structures in a given position when the motor is in an off state (i.e., when power is not applied and the motor is not in use). There are various reasons why it may be desirable to provide a braking mechanism in an electric motor.
For example, in one typical application, an electric motor is used to raise and lower a garage door. Heavy springs are used to reduce the force required to raise the door and to help keep the door in the raised position. However, variations in installation, the spring constant of the heavy spring, and the effects of aging often result in a spring force that is insufficient to maintain the door in the required position.
Manufacturers may require that the electric motor used to raise and lower the garage door be configured to hold a specified portion of the weight of the door in the raised position. For example, in the off state, an electric motor can be required to hold a certain amount of weight (e.g., a ninety-pound negative force) applied to the door after it has been raised.
The gear ratio in the drive train and the natural reluctance for the shaft of the motor to rotate in the off state (including the forces exerted by the brushes on the commutator and the magnets) provide some of the necessary resistance to movement when the motor is in the off state. However, these forces can be inadequate in some applications to maintain the load in the desired (i.e., raised) position.